Since the mid-1980's, three-dimensional (3D) films have greatly increased in popularity worldwide. 3D films enhance an illusion of depth perception, such that images in the film can appear to a viewer to extend in and out of a projection screen. Stereoscopic imaging devices are commonly used for playing 3D films, and in the past, have utilized dual projection systems with passive polarization. However, current stereoscopic imaging devices now utilize a single digital projection source combined with an active polarizing modulation device. For example, the single digital projection source can alternately project right and left eye frames. The active polarizing modulation device then alternately polarizes each frame using linear or circular polarization. Viewers wear glasses with oppositely polarized lenses to experience three-dimensional features that appear to extend in and out of a polarization-preserving projection screen.
However, while offering substantial quality benefits in comparison with dual projection systems, stereoscopic imaging devices with a single projection source emit images with substantially reduced brightness. Specifically, the light from a projection source must be linearly pre-polarized for the polarization modulating device to function. Other factors that contribute to the loss of light include the duty cycle of the projected left and right frames, dark time, white point calibration, reflective and transmissive surface losses, and polarization inefficiencies.
Attempts to increase image brightness of current stereoscopic imaging devices have been made. For instance, U.S. Pat. No. 7,857,455, to Cowan, discloses a multiple path stereoscopic projection system to enhance brightness of stereoscopic images perceived by a viewer. The system includes a polarizing splitting element, a reflector, a retarder, and a polarization modulator. Light received by the stereoscopic projection system is split into a primary path and a secondary path. The reflector and retarder are typically located in the secondary path, while the polarization modulator is located within at least the first path. The stereoscopic projection system includes multiple parts with exposed-to-air surfaces that can be difficult to clean, and which can reduce the quality and brightness of the images and the lifespan of the system if left uncleaned. Therefore, a stereoscopic imaging device with fewer parts and exposed surfaces is beneficial to maintain image quality and increase the life of the device.
Further, U.S. Pat. No. 7,905,602, to Schuck, discloses a polarization conversion system that is located in a randomly-polarized light path emitted by a projector. The polarization conversion system includes a polarizing beam splitter, a polarization rotating element, a reflecting element, and a polarization switch. The beam splitter separates p- and s-polarized light. The p-polarized light is directed on a first path to the polarization switch, while the s-polarized light is directed on a second path, passed through the polarization rotating element and transformed to p-polarized light before reaching the reflecting element which directs the now p-polarized light to the polarization switch. Additionally, the conversion system includes a telephoto lens pair to control magnification, distortion, and imaging properties of the first light path. The numerous parts and exposed-to-air surfaces of the polarization conversion system can be difficult to clean and expensive to maintain. However, without cleaning and maintenance, the quality and brightness of the images deteriorates and the life span of the conversion system is reduced.
Currently, a polarization conversion system with fewer parts and exposed surfaces is needed to increase quality and brightness of stereoscopic images while decreasing maintenance and increasing the life of the system.